This invention relates in general to cathodic arc deposition apparatus and, more particular to such an apparatus including means for separating metal droplets from the ion stream.
A number of different methods have been developed for depositing materials, generally metals, in the form of particles or ions onto a target surface to form an adherent, uniform coating. Among these are thermal deposition, cathode sputtering, chemical vapor deposition. While useful in particular applications, these methods suffer from several problems, including tending to coat other system surfaces than the target with the material being deposited, requiring frequent cleaning, contamination problems when the coating material is changed and a waste of often expensive coating material. Generally, these processes require that the target surface be heated to a very high temperature which often damages the target material, especially when the target is an organic material or an organic matrix composite material. The high deposition temperatures also lead to thermal stresses that may cause coating delamination.
Vacuum arc deposition has a number of advantages for coating difficult materials, such as refractory metals, onto targets. Vacuum arc deposition involves establishing of an arc, in a vacuum, between a cathode formed from the coating material and an anode, which results in the production of a plasma of the cathode material suitable for coating. The process does not involve gases, making control of deposition rate easier and simplifies changing coating materials. Typical vacuum arc deposition systems are described in U.S. Pat. Nos. 3,566,185, 3,836,451 and 4,714,860. Vacuum arc deposition, often referred to as cathodic arc deposition, is used commercially, typically to produce titanium nitride coatings on tooling.
Cathodic arc deposition, unfortunately, generates droplets of metal along with the metal ion plasma. These droplets, often called macro-particles, typically have diameters of from about 1 to 50 micrometers. The droplets travel outward from the cathode surface at such velocities that they often stick to the surface to be coated. Thus, cathodic arc coatings are often contaminated with macro-particles that adhere to the target surface, or leave holes where they once clung but have since been removed. The adhering macro-particles have little affect on wear resistance but each hole represents a site for corrosion to commence.
Thus, there is a significant, continuing, need for methods and apparatus to prevent or reduce the deposition of macro-particles while forming uniform, adherent metal or metal compound coatings on target surfaces.